1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to multi-piece golf balls.
2. Description of the Related Art
Golf players' foremost requirement for golf balls is flight performance. Flight performance correlates with the resilience performance of a ball. High resilience performance increases an initial speed, thereby achieving a long flight distance. In light of flight performance, a golf ball with high resilience performance is desired. In addition, generally, golf players prefer soft feel at impact.
Spin performance also correlates with flight performance. Flight at a low spin rate results in a proper trajectory, thereby achieving a long flight distance. In light of flight performance, a golf ball with high resilience performance and to which spin is difficult to impart, is desired.
The diameter of a core formed from a rubber composition correlates with the resilience performance of a ball. When the diameter of the core is large, the resilience performance of the ball is high.
JPH08-336617 (U.S. Pat. No. 5,688,595, US2003/109333, US2002/034987) and JPH09-56848 (U.S. Pat. No. 5,725,442) disclose multi-piece solid golf balls with a four-layer structure. These publications describe that a spin rate is suppressed and a flight distance is increased due to the rigidity distribution of a ball, and the like.
On the other hand, there has been an attempt to enhance resilience performance due to the material of an outer layer. JP No. 3767683 (US2002/099120) and JP No. 3729243 (U.S. Pat. No. 6,962,951, US2005/256269) disclose golf ball materials that include an ionomer and an aliphatic acid as essential components. JP2002-239033 (US2002/173380) discloses an inner cover in which an ionomer resin is blended with a styrene elastomer.
JP2006-500995 (US2004/132552) discloses an ionomer cover that is flexible and elastic. This ionomer cover includes: a carboxylate functionalized terpolymer with a weight average molecular weight of 80,000 or greater and 500,000 or less; and carboxylate functionalized ethylene with a weight average molecular weight of 2,000 or greater and 30,000 or less.